1. Field of the Invention
The present invention relates to a process for the preparation of a phosphor, and more particularly, to a process for the preparation of a phosphor comprising firing a mixture of starting materials for the phosphor under a reducing atmosphere.
2. Description of the Prior Art
There are known a variety of phosphors, and also known are various processes for the preparation of phosphors. A process for the preparation of a phosphor which comprises preparing a powdery mixture of starting materials for the phosphor by mixing and, if necessary, by subjecting the mixture to pulverization, and then firing the mixture of starting materials for the phosphor.
In the above-mentioned process for the preparation of a phosphor comprising firing a mixture of starting materials for the phosphor, the valence of an activator element contained therein is, in a certain case, reduced to the desired valence (to a lower valence than that for the activator element contained in the mixture of starting materials for the phosphor) in the firing procedure, for instance, as in the process for the preparation of a phosphor having a divalent europium as the activator. In such a case, the reduction of valence of the activator element is accomplished by carrying out the firing of the mixture of starting materials for the phosphor under a reducing atmosphere.
When a mixture of starting materials for a phosphor is fired under a reducing atmosphere as in the above-described case, the reducing atmosphere can be formed by introducing a reducing gas such as gaseous carbon dioxide containing carbon monoxide (CO) or gaseous nitrogen containing hydrogen (H.sub.2) into a firing system such as a firing furnace. Accordingly, the reduction of the activator element contained in the mixture of starting materials for the phosphor is carried out by action of CO or H.sub.2 introduced into the firing system. In this reduction method, however, there occurs such a problem that uniform reduction of the activator element can be hardly accomplished in the firing system. More in detail, the activator element present on the surface of the powdery mixture of the starting materials can be reduced rapidly upon direct contact with the reducing gas, while the activator element present inside of the mixture of the starting materials cannot be easily reduced. Accordingly, the reduction of the activator element is likely made ununiform.
Also known is a method utilizing powdery carbon (C) as a reducing agent for providing a reducing atmosphere to the firing system. This method is carried out, for instance, by placing the powdery carbon on a mixture of starting materials for a phosphor charged in a container for firing and then submitting them to the firing. After the firing is complete, the residual carbon in the container is separated and removed from the resulting phosphor. However, even in this method, a portion of the activator element being present out of contact with carbon, for instance, a portion of the activator element placed in the bottom of the container, cannot be sufficiently reduced. In order to avoid such ununiform reduction of the activator element, there can be offered a method involving sufficiently mixing the powdery carbon with the mixture of the starting materials prior to the firing thereof. However, in this case, it is difficult to separate the residual carbon from the resulting phosphor after the firing and, as a result, such a problem that a considerable amount of carbon is likely incorporated into the phosphor as impurity arises.
Accordingly, in the above-described conventional methods of firing and reducing a mixture of starting materials for a phosphor containing an activator element, it is difficult to obtain a phosphor which gives emission of sufficiently high luminance and/or shows a emission color of high purity, because the reduction of the activator element is ununiform or the impurities are present in the resulting phosphor.
As a method to solve the problems occurring in the conventional firing and reducing methods as described above, Japanese Patent Provisional Publication No. 57(1982)-67682 discloses a method of adding to the starting materials for a phosphor an organic compound decomposable in the firing procedure to produce a gas containing a reducing gas, in place of a method of introducing a reducing gas into a firing furnace. In the above publication, oxalic acid and dimethyl oxalate are mentioned as the organic compounds employable in the method and also mentioned are procedures of mixing the organic compound with dry starting materials for a phosphor under dry conditions by means of a mixing apparatus such as ball mill.
However, since the starting materials for a phosphor are mixed with a solid organic compound such as oxalic acid or dimethyl oxalate in the above-mentioned method, a sufficiently uniform mixture can be hardly obtained. Even if a sufficiently uniform mixture of these dry solid materials is obtained, still the activator element positioned on the surface of the powdery starting materials only is in direct contact with the solid organic compound serving as a reducing agent. As a result, it is difficult to achieve uniform reduction of the activator element. Further, if the organic compound as described above is employed, carbonaceous product produced by thermal decomposition thereof likely remain in the resulting phosphor. Therefore, even in this method, it is difficult to obtain a phosphor satisfactorily improved in the luminance of emission.